The invention relates to a device for the multi-channel measurement of weak variable magnetic fields having field strengths below 10.sup.-10 T, and in particular below 10.sup.-12 T. The device contains a superconducting quantum interference device in each channel, and a gradiometer consisting of superconducting coils and superconducting connecting elements between the quantum interference device and the gradiometer. There is also a coupling transformer and connecting lines, in addition to electronic equipment for the evaluation, processing and presentation of the information obtained at the quantum interference devices. The invention also relates to a method for manufacuturing this measurement device.
The use of superconducting quantum interference devices, which are generally referred to as "SQUIDs" (abbreviation for "Superconducting Quantum Interference Devices), for the measurement of very weak magnetic fields is generally known ("J. Phy. E:Sci. Instrum.", Vol. 13, 1980, Pages 801 to 813; "IEEE Transactions on Electron Devices", Vol. ED-27, No. 10, October 1980, Pages 1896 to 1908). The preferred field of application for these devices is medical engineering and in particular magnetocardiography and magnetoencephalography. The magnetic cardiac or brain waves that occur in these sectors have field strengths that are located in the range from 50 pT to 0.1 pT ("Biomagnetism-Proceedings, Third International Workshop on Biomagnetism, Berlin 1980", Berlin/New York 1981, Pages 3 to 31).
A device for the measurement of biomagnetic fields of this kind includes the following principal components:
1. A SQUID, acting as a current sensor;
2. A flux transformer, a coil arrangement acting as a field-to-current transducer for sensing the field;
3. Electronic devices for collecting and processing signals;
4. Screening for the geomagnetic field and external interference fields; and
5. A cryogenic system for the superconducting components.
Measuring devices of this type are known (S.H.E. Corporation, San Diego, USA/S.H.E. GmbH, D-5100 Aachen).
In corresponding measurement devices with a one-channel design, the magnetic field to be investigated is coupled inductively through a coil arrangement made of superconducting wire into a circuit consisting of a radio-frequency (RF) SQUID with one Josephson contact. Gradiometers of the first or higher orders are constructed by combining one sensor coil with one or more compensation coils. With gradiometers of this type, it is possible, with the right kind of manual adjustment, to suppress almost entirely the three components of a homogenous magnetic field in the vicinity of the coils and/or the portion of such field with homogenous gradient. Also, the biomagnetic near field, which is still strongly non-uniform in the vicinity of the gradiometer, can be selectively obtained. The RF SQUID is also inductively coupled with a tank circuit, whose high-frequency voltage is modulated in phase or amplitude by the input signal. Generally, the operating point of the RF SQUID is maintained by negative feedback through an additional compensation coil, and the compensation current is used as a signal to be evaluated electronically.
The RF SQUIDs used in these units have a characteristic noise signal (cf., for example, "SQUID Superconducting Quantum Interference Devices and Their Applications", Berlin/New York 1977, Pages 395 to 431). Therefore, to measure the above-mentioned extremely weak magnetic fields, it is necessary to compute an average value for the signal at the individual measuring points with the aid of a large number of individual measurements. To obtain a spatial field distribution, it is necessary to take measurements one after the other at various points within the area to be investigated. In a measurement procedure of this type the field data will not remain coherent over the requisite measuring time, and, in addition, measuring times that are unacceptable for clinical purposes will occur.
To overcome these problems a multi-channel measurement device has been used instead of the familiar one-channel device. In the multi-channel device each channel has an RF SQUID, a tunable superconducting gradiometer and connecting elements between the SQUID and the gradiometer, which includes a coupling transformer and leads. Substantial time is lost in the utilization of this device, however, because the individual channels must be tuned to each other. Typically the gradiometer and the SQUID with its coupling transformer, are each arranged on their own carrying substrate and are connected with one another via detachable leads. This kind of connection, however, does not have the possibility of providing a constant tuning of the respective flux transformer in advance. Instead, it is necessary prior to every measurement to adjust all the channels. This adjustment can be difficult and time consuming because all the channels interact with one another. In addition, mutual interference of the RF circuits is unavoidable in this arrangement. If the gradiometer is dispensed with, an RF screening can, however, be achieved (cf. for example, "Physica", Volume 197,B, 1981, Pages 29 and 30). But even if an RF decoupling can be provided in some other way, the above-mentioned tuning problem remains.
A measurement device with three RF SQUIDs and only one tank circuit on a common substrate is disclosed in the publication "Cryogenics", December 1981, Pages 707 to 710. However, there are technical problems with this device. All the signal channels are combined into one high-frequency channel which provides the possibility of mutual interference. Additionally, the individual SQUID elements must be tuned to one another with regard to their critical current. Finally, the maximum number of elements that can be controlled in practice is therefore believed to be about 10.
In addition to the RF SQUIDs, each with a Josephson contact, that have been described above, direct current (DC) SQUIDs which include two Josephson contacts are also known. These DC SQUIDs can be designed so that they have an extremely small noise signal ("IEEE Transactions on Magnetics", Vol. MAG-17, No. 1, January 1981, Pages 395 to 399). The use of a modularly constructed system with 5 SQUIDs of this type for the measurement of biomagnetic fields is suggested. However, even though this device eliminates the problem of mutual RF interference, the above-mentioned tuning problem still exists.